GB2336257A - Detecting operational faults in a three phase alternator - Google Patents

Abstract

An alternator charging a vehicle battery has an arrangement for detecting the occurrence of an open or shorted phase, an open or shorted diode, and an open or shorted field. The stator voltages are measured and summed, and then AC coupled and rectified to produce positive pulses. The resulting signals are averaged and differences are detected by comparators. If signals differ by more than 6%, a fault indicating LED is lit. Additionally, a window comparator compares an average of the three phase-to-ground voltages with 6 volt and 3 volt reference levels. If the average lies outside the window, a fault is again indicated.

Description

2336257 1 A METHOD AND APPARATUS FOR DETECTING A MULTITUDE OF FAULTS IN AN

AUTOMOBILE CHARGING SYSTEM The present invention relates to an apparatus for detecting faults in an automotive charging system. More specifically, the present invention relates to an apparatus for detecting a multitude of faults in an automotive charging system while the system is operating under all normal operating conditions.

Automobile alternators suffer from a variety of known faults or malfunctions. These various faults cannot normally be detected from a mere inspection of the alternator. moreover, these faults cannot normally be detected during normal operating conditions without an apparatus to assist in the detection of these faults.

Various devices have been designed to detect the presence of an alternator fault. One known method of detecting an alternator fault is to continuously monitor a single phase voltage output from the alternator. Typical alternator faults include open phases, shorted phases, open diodes, shorted diodes, an open field, or a shorted field.

Numerous methods have been developed for detecting some of these alternator f aults.

Patent No.

For example, U. S.

5,252,926 discloses a method of distinguishing between a break in the wire monitoring the single phase voltage output of the alternator and an actual broken belt. This device is not intended to detect any of the alternator faults listed above. Some prior art systems, such as that disclosed in U. S. Patent

No. 4,623,833, monitor an alternator for some of the fault conditions above, such as (1) broken drive belt; (2) over-voltage to the battery; and (3) undervoltage to the battery. This system, however, cannot detect all of the faults above, and those it can detect, it cannot detect under all normal operating conditions.

Other prior art systems disclose apparatus for detecting only some of the above mentioned alternator faults under normal operating conditions. For example, U.S. Patent No. 4,613,808 discloses a system for monitoring an alternator's single phase output for peaks above a set threshold. U.S. Patent No. 4,348,629 discloses a system for comparing an alternator's output to preset high and low references and then looks for asymmetry in the three alternator phases. None of these - 3 devices can detect all of the above f aults under all normal operating conditions.

Moreover, some of these prior art devices that can detect some of the above alternator faults utilize complex circuity that is expensive to manufacture. Accordingly, applicants have recognized that a system for detecting all of the following alternator faults under all normal operating conditions, namely, any open phase, any shorted phase, any open diode, any shorted diode, an open field, and a shorted field, that utilizes a simpler operating circuit and that is cheaper to manufacture is needed.

The apparatus of the present invention can detect all of the following fault conditions: any open phase, any shorted phase, any open diode, any shorted diode, an open field, and a shorted field under normal operating conditions. The fault detection circuit is to be incorporated into a voltage regulator as part of a battery charging system. Such a charging system includes an alternator with a field winding whose current is controlled by the voltage regulator and a three phase stator winding feeding induced currents to a diode bridge, producing DC voltages to charge a battery. The fault detection circuitry is connected to each of the stator windings to measure the voltages across each winding. The stator voltages are summed, AC - 4 coupled, and rectified to produce positive pulses and then averaged so that any difference can be detected by comparators in the circuitry. If these signals are equal, the system is in a no fault condition. If the signals are unequal, the system is in a fault condition, and the LED is illuminated.

Additionally, because field faults do not produce an imbalance in these signals and shorted diode faults do not develop large enough variances in the signals under all normal operating conditions to trigger a fault condition, i.e., greater than 6% variance, a window comparator is added. The window comparator compares an average of the three phase to Bvoltages to a 6V reference and a 3V reference. If the value is not in that range, the system is placed in a fault condition and the LED is illuminated red.

The invention will now be described by way of example and with reference to the accompanying drawings wherein:

is Figure la is a schematic block diagram of a fault detection circuit in accordance with the preferred embodiment of the present invention.

Figure ib are graphs illustrating various waveforms corresponding to points in Figure la of the preferred embodiment of the present invention.

Figure 2 is a schematic diagram of the fault detection circuit in accordance with the preferred embodiment of the present invention.

Figure 3 is a schematic diagram of the input summing circuit in accordance with the preferred embodiment of the present invention.

Figure 4 (a) is a schematic diagram of a three winding stator and rectifying bridge illustrating normal operation of the preferred embodiment in accordance with the present invention.

Figure 4 (b) is an oscilloscope picture illustrating the waveforms of the voltages (VA + VB) /2, (V,, + V,) /2, and (V. + V,) /2 in accordance with the embodiment shown in Figure 4(a).

Figure 5 (a) is a schematic diagram illustrating an alternator open phase fault in accordance with a preferred embodiment of the present invention.

Figure 5 (b) is an oscilloscope picture illustrating the waveforms of the voltages (VA + V3) / 2, (V, + Vc) / 2, - 6 and (VC + Vj /2 in accordance with the embodiment shown in Figure 5(a).

Figure 6 (a) is a schematic diagram illustrating an alternator short phase fault in accordance with a preferred embodiment of the present invention.

Figure 6 (b) is an oscilloscope picture illustrating the waveforms of the voltages (V,, + VJ/2, (Vs + VC)/2, and (VC + V.)/2 in accordance with the embodiment shown in Figure 6(a).

Figure 7(a) is a schematic diagram illustrating an alternator short negative diode fault in accordance with the present invention.

Figure 7 (b) is an oscilloscope picture illustrating the waveforms of the voltages (V, + V,)/2, (VB + VC) / 2, and (VC + vJ/2 in accordance with the embodiment shown in Figure 7(a).

Figure 8(a) is a schematic diagram illustrating an alternator short positive diode fault in accordance with a preferred embodiment of the present invention.

Figure 8 (b) is an oscilloscope picture illustrating the waveforms of the voltages (VA + Vs) /2, M + VC) /21 and (V. + VA) /2 in accordance with the embodiment shown in Figure 8(a).

Figure 9 (a) is a schematic diagram illustrating an alternator open negative diode fault in accordance with the present invention.

Figure 9 (b) is an oscilloscope picture illustrating the waveforms of the voltages (V, + V,) /2, (VB + Ve)/2, and (VC + VA) /2 in accordance with the embodiment shown in Figure 9(a).

Figure 10 (a) is a schematic diagram illustrating an alternator open positive diode fault in accordance with the present invention.

Figure 10(b) is an oscilloscope picture illustrating the waveforms of the voltages (V. + Vs) /2, M + VC) /2, and WC + VA) /2 in accordance with the embodiment shown in Figure 10(a).

is Figure la is a block diagram of an embodiment of circuitry for detecting various faults in an automotive charging system while the automotive charging system is operating under normal operating conditions. Figure ib graphically illustrates the associated waveforms corresponding to the numbered points in Figure la. Figure 2 illustrates an embodiment of the circuitry of a preferred embodiment. Normal operating conditions are defined as an alternator speed of approximately 1500 RPM or more and a minimum of 5 amps of current being drawn from the alternator. The circuitry of Figure 2 is only part of the automotive charging system of the present invention.

Referring to Figure 2, the automotive charging system consists of electronic circuitry 10, an alternator with a voltage regulator (not shown), a battery (not shown), and an electrical load (not shown). The various faults that the circuitry of Figure 2 is designed to detect include any open phase, any shorted phase, any open diode, any shorted diode, an open field, and a shorted field. The circuitry is designed to detect these faults under normal operating conditions.

Turning to Figure 4 (a), a stator 12 of a three phase alternator in accordance with the invention is illustrated. The stator 12 feeds current to a diode rectifying bridge 14 which converts the current from AC to DC in order to charge a battery 16.

present The current is fed to the rectifying bridge by connections leading from points A, B, and C. The rectifying bridge 14 is made of three positive diodes 18 and three negative diodes 20. One positive diode 18 and one negative diode 20 are associated with a respective connection from points A, B, and C.

In order to ensure that the stator 12 is working properly, the voltages of each winding of the alternator are measured. The voltage at point A with respect to battery ground is VA. Voltages V. and Vc are similarly defined with respect to battery ground. The line to line voltage, V;,,, across the winding 22 is equal to the voltage V. minus the voltage V.. The line to line voltage, VBcr across the winding 24 is equal to the voltage V. minus the voltage Vc. The line to line voltage, V... across the winding 26 is equal to the voltage V. minus the voltage V..

When the alternator system of the present invention is operating properly, i.e., without any faults, the line to line voltages are balanced (VA,, = VBC = VCA) Moreover, when the apparatus of the present invention is operating properly, the RMS values across the windings 22, 24, 26 are equal under all normal operating conditions.

However, when a system fault occurs, such as an open or shorted phase or diode, the line to line voltages across the windings 22, 24, 26 becomes unbalanced (VA3;'V,,;VcA) and their RMS values become - 10 unequal under all normal operating conditions. In order to compare the stator voltages, the circuitry of the preferred embodiment sums the phase to battery ground voltages. If the line to line voltages are unequal, these voltage sums will also be unequal. The apparatus and method of implementing the voltage sums is far less complex than methods utilizing the line to line voltages which require three operational amplifiers and twelve resistors.

Turning now to Figure 3, an input summing circuit 28 in accordance with the preferred embodiment of the present invention is illustrated. The input summing circuit 28, as shown in Figure 3, consists of an electrical connection to two stator terminals (not specifically shown but represented by VA and V. respectively), two resistors 30, 32, and a battery ground 34. VA represents the stator point A (Figure 4 (a)) voltage referenced to the battery ground 34. VB represents the stator point B (Figure 4 (a)) voltage referenced to the battery ground 34. The voltage is then summed by measuring it across point 36 which is located between the resistors 30 and 32 and the battery ground 34. This voltage is then determined by the following equation: (VA + VJ/ 2. It can be seen that by simply implementing a total of 6 resistors as shown in Figure 2, the summed values of all three stator terminals can be easily determined to monitor alternator faults that occur during normal operating conditions.

11 - is Figure 4 (b) illustrates the waveform of the alternator when it is operating properly. As Figure 4 (b) illustrates, when the alternator is operating properly, the summed voltages across each stator terminal are balanced. Also, the RMS values of the summed voltages are equal. When the alternator is functioning properly, as shown by typical waveforms in Figure 4 (b), the RMS values for channel 1, 2, and 3 are equal (to within. 08V) and were measured at 7.824V, 7.788V, and 7.84OV respectively. The waveforms for Figures 4 (b). through 10(b) were traced on an oscilloscope with channels 1 Grepresented by reference number 38), 2 (represented by reference number 42), and 3 (represented by reference number 40) each having a voltage of two volts/division with a time factor of 1.00 milliseconds/division.

Turning now to Figure 5 (a), which schematically illustrates an alternator in an open phase fault condition. The open phase is generated by a break in connection in one of the windings. This break is schematically represented by reference number 44. Figure 5(b) illustrateis typical waveforms that are generated when the summed voltages were measured when the alternator had an open phase fault in accordance with the preferred embodiment disclosed herein. When this fault condition was run and the waveforms generated, the RMS values for channels 1, 2, and 3 were 8.044V, 7.56OV, and 7.672V respectively.

- 12 is Figure 6 (a) schematically illustrates an alternator in a short phase fault condition. The short phase is generated by a crossing or connection across two of the stator terminals. This short is illustrated by line 46 which connects stator terminal A 48 to stator terminal B 50. Figure 6 (b) illustrates typical waveforms that are generated when the voltage sums were measured when the alternator had a short phase fault. When this fault condition was run and the waveforms generated, the RMS values for channels 1, 2, and 3 were 8.164V, 8.076V, and 6.132V respectively.

Figure 7 (a) schematically illustrates an alternator in a short negative diode fault condition. This fault condition is illustrated by the line 52 which shorts the negative diode 20 to battery ground. Figure 7(b) illustrates typical waveforms that are generated when the summed voltages were measured when the alternator had a short negative diode fault in accordance with the preferred embodiment disclosed herein. when this fault condition was run and the waveforms generated, the RMS values for channels 1, 2, and 3 were 3.528V, 5.432V, and 3.684V respectively.

Figure 8(a), schematically illustrates an alternator in a short positive diode fault condition. This fault condition is illustrated by the line 54 which shorts the positive diode 18 to the positive terminal of the battery. Figure 8(b) illustrates typical waveforms that are generated when the summed voltages were 13 measured when the alternator had a short positive diode fault in accordance with the preferred embodiment disclosed herein. When this fault condition was run and the waveforms generated, the RMS values for channels 1, 2, and 3 were 11.588V, 9.576V, and 11.584V respectively.

Figure 9(a), schematically illustrates an alternator in a open negative diode fault condition. This fault condition is illustrated by the break in line 55 leading from the anode of the negative diode 20. Figure 9(b) illustrates typical waveforms that are generated when the summed voltages were measured when the alternator had a open negative diode fault in accordance with the preferred embodiment disclosed herein. When this fault condition was run and the waveforms generated, the RMS values for channels 1, 2, and 3 were 7.032V, 6.28OV, and 7.50OV respectively.

Figure 10(a), schematically illustrates an alternator in a open positive diode fault condition. This fault condition is illustrated by the break in line 56 leading from the cathode of the positive diode 18. Figure 10(b) illustrates typical waveforms that are generated when the summed voltages were measured when the alternator had a open positive diode fault in accordance with the preferred embodiment disclosed herein. When this fault condition was run and the waveforms generated, the RMS values for channels 1, 2, and 3 were 8.40V, 9.14V, and 8.97V respectively.

14 - is It is clear from Figures 5 through 10, discussed in detail above, that the RMS values of the summed voltages are equal only when there is no fault present. Circuits for generating and calculating the RMS values of these signals are prohibitively expensive for this application. The average values of the summed voltages (e.g. (V. + VB) /2 are not in all cases unequal due to de bias on the waveforms. As shown in Fig. 5 (b), the average values for the open phase fault condition can be seen to be about 7V.

Accordingly, the circuitry of the present invention, provides an inexpensive filter that will, like the RMS operation, distinguish all of the fault conditions from non-fault conditions. Referring back to Figure 2, a schematic of the fault detection circuit 10 in accordance with a preferred embodiment of the present invention is shown. The input summing circuit 28, described previously in Figure 3, is illustrated here as part of the overall circuit.

Turning to Figure 2, stator terminal a (S.T.A.) is connected to and feeds AC current to the circuit at point 58. Stator terminal b (S.T.B.) is connected to and feeds AC current to the circuit at point 60. Stator terminal c (S.T.C.) is connected to and feeds AC current to the circuit at point 62. S.T.A. is connected to one end of a resistor 64 and to one end of another resistor 66. The resistor 64 is connected at its other end to one end of a resistor 68 which is connected at its other is end to point 60 which receives the AC current from S. T. B. That point 60 is also connected. to one end of another resistor 70.

The other end of the resistor 66 is connected to an end of a resistor 72 which is connected at its other end to point 62 which receives the AC current from S.T.C. That point 62 is also connected to one end of a resistor 74. Resistors 64, 66, 68, 70, 72, and 74 are all In addition to being connected to the resistor 68, the other end of the resistor 64 is also connected to the cathode end of a zener diode 76 and to a capacitor 78. In addition to being connected to one end of the resistor 72, the other end of the resistor 66 is also connected to the cathode end of a zener diode 80 and to a capacitor 82. The resistor 70 is connected to one end of the resistor 74, to the cathode end of a zener diode 84, and to a capacitor 86. The capacitors 78, 82, and 86 act as filters to AC couple the (VA + VJ/2 signals and remove the DC bias. The zener diodes 76, 80, and 84 provide load dump protection to the circuit and protect against large surge voltages. The capacitors are preferably commercially available 3.3 micro farad capacitors.

The anode end of zener diodes 76, 80, and 84 are all referenced to battery ground 88. The zener diodes 76, 80, and 84 are preferably commercially available and bear part no. MMSZ524W. The other end of each capacitor 78, 82, and 86 is connected to a respective preferably 4.32k resistors 16 resistor 90, 92, and 94, each of which is referenced to battery ground 88, and to a respective rectifying diode 98, and 100 commercially resistors of The purpose is to remove (VA + VB) /2 is 96, 98, and 100. The rectifying diodes 96, of the preferred embodiment are also available and bear part no. BAS-16, and the the preferred embodiment are 10k resistors. of the rectifying diodes 96, 98, and 100 the negative portions of the AC coupled signals (see Fig. 1).

The rectifying diode 96 is connected to a resistor 102, which is referenced to battery ground 88, and to another diode 104. The rectifying diode 98 is also 106, which is referenced to to another diode 108. The connected to a resistor battery ground 88, and. rectifying diode 100 is also connected to a resistor 110, which is referenced to battery ground 88, and to another diode 112. The diodes 104, 108, and 112 are the preferably of the same kind as rectifying diodes 96, 98, and 100. The resistors 102, 106, and 110 are preferably 47.5k resistors.

The diodes 104, 108, and 110, resistors 114, 116, 118, 120, 122, 124, 126, 128, and 130, and capacitors 132, 134, and 136 act as a weighted filter and average the positive portion of the signal received from rectifying diodes 96, 98, and 100. The capacitors 132, 134, and 136 charge with an RC time constant of 47 milliseconds (114132). These same capacitors 132, 134, and 136 discharge with a 240 millisecond RC time - 17 is constant (120+126) 132. This arrangement gives filtering weighted with peak detection to maximize the inequality of the signals, thus allowing for the detection of any fault conditions.

As shown in Figure 2, the diodes 104, 108, and 112 are each connected to the resistors 114, 116, and 118 respectively. The resistor 114 is connected at its other end to the capacitor 132. The resistor 116 is connected at its other end to the capacitor 134. The resistor 118 is connected at its other end to the capacitor 136. In the preferred embodiment, the resistors 114, 116, and 118 are 10k resistors and the capacitors 132, 134, and 136 are 4.7 ufarads. The other end of the capacitors 132, 134, and 136 are referenced to battery ground 88.. Each of the capacitors 132, 134, and 136 are also connected to the resistors 120, 122, and 124. The other end of resistor 120 is connected to the resistor 126, which is referenced to battery ground 88, and to a negative input 137 of a comparator 138 and a negative input 139 of a comparator 140. The other end of the resistor 122 is connected to the resistor 128, which is referenced to battery ground 88, and to a negative input 141 of a comparator 142 and a negative input 144 of a comparator 146. The other end of the resistor 124 is connected to the resistor 130, which is referenced to battery ground 88, and to a negative input 148 of a comparator 150 and a negative input 152 of a comparator 154.

- 18 is The other end of the resistor 120 is connected to a positive input 156 of the comparator 154 and a positive input 162 of the comparator 142. The other end of the resistor 122 is connected to a positive input 158 of the comparator 150 and a positive input 166 of the comparator 138. The other end of the resistor 124 is connected to a positive input 167 of the comparator 146 and to a positive input 168 of the comparator 140. The resistors 120, 122, and 124 are preferably 3.01k resistors and the resistors 126, 128, and 130 are preferably 47.5k resistors.

The filtered signals discussed above, are divided by the resistors 120, 122, 124, 126, 128, and 130. The r a t i o f or S. T. A. ( p o i n t 5 8) i S V[ (A+B) /2avg] [126/ (120+126)l = 0. 94 V[ (A+B) /2avg] where it is understood that V[M+W/2avg] represents the signal with ac coupling, AC coupling, rectification and fitting as discussed above. The comparators 138 and 142 then implement the following comparisons:

138: V[(A+C)/2avg] >.94V[(A+B)/2avg] (no fault) V[M+C)/2avg] <.94V[M+ B)/2avg] 142: V[(A+W/2avg] >.94V[(C+A)/2avg] V[(A+W/2avg] <.94VE(C+M/2avg] Fault = 1 Fault = 0 Fault = 1 Fault = 0 If the comparator 138 or 142 determines that V[(A+ 13)/2avg] varies by more than 6% from V[(C+A)/2avg] then the fault output is set to zero. The comparators 140, 146, 150, and 154 implement similar comparisons so that all of the averaged signals are compared for a is variance of more than 6-15. If fault signal 170 is set to logic state 1 (1OV), an LED (not shown) would be turned green. The fault light is preferably a LED indicator and when the alternator is functioning properly the fault signal 170 is set to logic state 1 (i.e., 1OV). If the fault signal 170 is set to logic state 0 (i.e., 0 volts), then one of the six faults described above, is detected. In the preferred embodiment, the comparators bear part no. TLC3391.

The shorted diode fault condition discussed above does not create voltage inequalities greater than 6% for all normal operating conditions. Moreover, the field faults (open or short) discussed above, do not cause a machine imbalance and the developed signals remain equal. Accordingly, a window comparator 200 is included to detect these conditions. The window comparator 200 averages the line to battery ground voltages from the stator terminals received at points 202, 204, 206 using the resistors 208, 210, 212, 214, 216, and 218 and the capacitor 220. A shorted diode or an open or shorted field pulls the average value out of its normal 3 to 6 volt range. A comparator 222 checks for V[(A+B+C)avg] > 6V and comparator 224 checks for V[(A+B+C)avg] < 3V. The zener diodes 226, 2219, and 230 are connected at their respective cathode ends to the resistors 208, 210, 212 and at the other end are referenced to battery ground 232.

- 20 is In operation, the comparator 222 has the filtered signal input into negative input 234 and a 6V reference 236 is input into positive input 238. The comparator 224 has the filtered signal input into negative input 240 and a 3V reference 242 is input into the positive input 244. All of the comparators are connected to the fault indicator 170 by a line 246. The 3V reference is formed from a voltage divider with resistor 250 connected to a regulated 1OV source, connected at its other end to resistor 252 which is in turn connected to battery ground 232. The 6V reference is similarly derived from the regulated 1OV source using resistors 254 and 256. Comparators 138, 140, 142, and 146 are bypassed by capacitor 258 from the 1OV source to the ground. Comparators 150, 154, 222, and 224 are bypassed by the capacitor 260 when from the 1OV source to ground.

The circuit described above and disclosed in Figure 2, is intended to be incorporated into a voltage regulator. Figure la illustrates in block diagram format the elements detailed in Figure 2. The corresponding waveforms of Figure ib further illustrate operation of the invention.

The fault signal developed can be used to light an LED, mounted on the voltage regulator, red for fault conditions and green for normal conditions. Alternatively, the fault signal feeds a lamp driver for the charging system dash mounted indicating lamp. The LED would indicate to an observer looking for problems 21 that there was a fault in the charging system. The operation of lighting the fault light is disclosed in copending application serial no. 08/542, 647 which is hereby incorporated by reference.

In an alternative embodiment, the circuit is built into a handheld instrument for trouble shooting an electrical system without the built in fault detection system. This circuit could also be integrated into an integrated circuit to reduce its size and cost.

Although the present invention has been described with reference to a specific embodiment, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims.

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Claims (10)

1. An apparatus for detecting a plurality of faults in the operation of an alternator having three phases, the apparatus comprising:

an averaging circuit connected to each phase of the alternator, the averaging circuit providing an output; a plurality of signal processing paths respectively connected to each phase of the alternator, each path comprising a summing circuit connected to a pair of phases of the alternator, an AC coupling circuit connected to the summing circuit, a rectifier connected to the AC coupling circuit, and a filter connected to the rectifier, the filter providing a filtered ouptut; a plurality of scaling circuits connected to scale each filtered output; a plurality of comparators connected to receive the plurality of scaled filtered outputs and the output from the averaging circuit, the plurality of comparators providing a comparison output; and an indicator connected to the plurality of comparators, the indicator being responsive to the comparison output.

2. The apparatus of Claim 1 wherein the scaling circuits provide a scaled filtered output approximately equal to 94k of the filtered output.

23 -

3. The apparatus of Claim 1 wherein the indicator is a light.

4. The apparatus of Claim 1 further comprising a window comparator connected to the averaging circuit.

5. A method of detecting a plurality of faults in the operation of an alternator having three phases, the method of comprising the steps of:

determining the differences in alternator line voltages between respective phases; determining a relative magnitude of an RMS value of line-to-line alternator voltages; comparing the relative magnitudes to each other; and indicating a fault when the relative magnitudes are not approximately equal.

6. The method of Claim 5 further comprising the step of indicating a fault when the relative magnitudes differ by more than 69g.

7. A method of detecting a plurality of faults in the operation of an alternator having three phases, A, B, C, the method comprising the steps of:

summing line voltages from phases A and B, B and C, and C and A to provide a respective summed signal; AC coupling each respective summed signal to provide an AC coupled output signal; 24 rectifying each AC coupled output signal to provide a rectified signal; filtering each respective rectified signal to provide a filtered signal; scaling each respective filtered signal to provide a scaled signal; comparing selected scaled signals to provide a fault signal; and indicating a fault in response to the fault signal.

8. The method of Claim 7 wherein the step of scaling the filtered signal comprises scaling each respective filtered signal to 94-. of the filtered signal.

9. The method of Claim 7 further comprising the steps of: averaging each of the line voltages A, B, and C to provide an averaged output; comparing the averaged output to a reference voltage; and indicating a fault if the average voltage does not approximately equal the reference voltage.

10. A f ault comprising:

detection circuit apparatus an alternator electrically connected to a battery, the alternator having a f irst phase A, a second phase B, and a third phase C, each phase having a tap; a plurality of summing circuits connected to pairs of taps from the alternator; a plurality of AC coupling circuits respectively connected to the plurality of summing circuits; a plurality of rectifiers respectively connected to the AC coupling circuits; plurality of filters connected to the rectifiers; plurality of scalers connected to the filters; plurality of comparators. connected to the scalers in a preselected manner; an averaging circuit connected to each phase tap and providing an average ouput to the comparators; and an indicator connected to the comparators.